Why should I take mushroom extracts on an empty stomach with vitamin C ?

This recommendation is not applicable to alcohol extracted Lion 's Mane, because that extract is optimized for the alcohol-solubles and is best taken with food.

The main active ingredient of most medicinal mushroom extracts are water-soluble beta-glucans, though. These are very large macro-molecules with a high molecular weight; in fact a type of dietary fiber.

When taken orally, intestinal absorptive cells (enterocytes) facilitate the transportation of beta-glucans and similar compounds across the intestinal cell wall into the lymph, where they begin to interact with receptors of the immune system, activating a wide spectrum of immunological effects.

Due to their size beta-glucans do not pass the intestinal wall easily. To increase the chances of beta-glucan entering the blood-stream it's common sense to have as little 'competition' as possible present. So no solid foods or fiber-rich smoothies !

Research found that the absorption / therapeutic effect of beta-glucans is improved when combined with vitamin C (ascorbic acid). The type or amount is not important: 20 - 50mg of the cheapest type will do. See e.g. this article - a Google search for 'beta-glucan vitamin C' will return many more results.

Vitamin C will also improve the absorption of iron.